stored energy

[firewater100]

I tryed to do a little search on tradgang but havnt quite found the answer.
     I realy like the info in traditonal bowhunter articals by blacky schwarz when he is testing bows . My question is how do you come up with the stored energy figuar of a given bow? I can do the draw force chart and crono but is there a formula to get the stored energy?

[wingnut]

stored energy is the area under the curve.  The easiest way is to total each inch up and divide by 12.  That will get you close

Mike

[O.L. Adcock]

What Mike said..It's not a "formula", it's a measurement....O.L.

[Billy]

I'm not 'into' the techy stuff; but I do like to know how you get the answers.

So, do you measure lineally the limbs and string, at full draw?

Or, the plane/area of the "triangle" - from anchor down to knock, up to the upper knock and back to anchor....?

Hope someone makes sense of that.

[O.L. Adcock]

Billy, you start at brace height. Pull it 1" and write down the weight...Draw it to 2", write that weight down...Keep going to what ever draw length you are checking it at. Add all those weights up, that's in/lbs, divide by 12 to get ft/lbs.

To get an accurate number takes accurate draw length measurements and an accurate scale. If it's off that error adds up at every inch so the end numbers can be WAY off without careful attention.

At 28", the better bows will store in the ball park of .92 ft/lbs per pound of draw force. The bigger that number is the better...O.L.

[Billy]

OK, I get it now! Thanks O.L.

[Cupcake]

I just did that with my latest bow and guess what.  It is storing 0.9115 ft/lbs per pound.  It sure seems to be fast but I have not tested it yet.
Kevin

[gordonf]

Okay, so how do you then figure out the efficency coefficent of a bow?

[O.L. Adcock]

After you find the stored energy, you shoot an arrow at the same draw length through a chrono. Calculate it's KE and divide that by the stored energy. Most glass/carbon bows will run 75-85%...O.L.

Here is a good site to make figuring KE easy:  

 http://home.att.net/~sajackson/archery.html

[gordonf]

Perfect. Thank you!

[firewater100]

Thanks very much guys, i assumed it was somthing simple i just did not have the clue.  Billy... like you said i not so techy either but i have built a couple bows and thought it might help give me hard evidence between each one i make and as i try to make improvements to the design. Not just feel in the hand.

Thanks agin

scott

[O.L. Adcock]

Scott, That's a good thing but the hard part is identifying what changes you made that caused any performance differences. Lots of variables, some known, some unknown, and many dismissed as not being a factor when they are. But asking "why" and digging for the answers is the first step!   ....O.L.

[Billy]

I'm glad you guys are here.
I'm glad we all see traditional bows and arrows as common ground.

Some guys are flight archers,some hunters, some have "bubba-tudes" and others "cerebral-tudes"; all share the stick and string.

Knowing, or at least being aware of; the hows and wherefores, makes me more willing to share my experiences.

Thanks for growing my knowledge, and oh yeah;
go out and fling more arras...
 

[Dartwick]

But dont get too carried away with "stored energy" itself. Its mostly a curiosity as best I can tell.
Although if you are designing and building bows its useful to to and understand the draw weights at  


As an shooter what really matters
-the peak draw weight
-that the bow is not stacking at peak draw weight(simply for easy of shooting)
-the velocity of a given arrow


NOTE: I know other things matter. But they arent as dirrectly related to stored energy.

[O.L. Adcock]

Stored energy and draw weight go hand in hand. What you don't want is a 50# bow storing 30ft/lbs when it should be 45.

When one sees one bow that out performs another, it can only happen if, one stores more energy then the other, or...one is more efficient then the other, or both....Those are the only factors effecting performance..."HOW" to get those things is the hard part....O.L.

[Dartwick]

Well not quite. Moving limb mass will effect performance too.

Thats why I say as an archer your better off just looking at the final draw wight and the speed at which they throw arrows(of the same weight).
Assuming there is no stacking issue to worry about.

EDIT: I missed that you mentioned efficiency. Arrow velocity gives you the end result of stored energy and efficiency, so most people only need to worry about it.

[O.L. Adcock]

"Moving limb mass will effect performance too."

Nope, it effects efficiency..If you have more stored energy and less efficiency, it can shoot the same or faster. Both go together, a lot of one does you no good without a lot of the other!   ....O.L.

[Dartwick]

Your trying to play semantics here - but you arent winning. Less stored energy decreases performance. Lower efficiency decreases performance.


Im not really sure why you are trying to defend the use of stored energy for archers as a useful measure.
Arrow velocity(assuming same arrow weight and peak draw weight)takes into account both efficiency and stored energy so is what shooters should be looking at if they are going to look at numbers.

[Jeremy]

Im not really sure why you are trying to defend the use of stored energy for archers as a useful measure.

End results are the results, but the why of it is in the underlying data.  Knowing why something is the way it is can give you a handle on what improvements you need to make.  Plus, some people just like being able to put a number on something  

[LEWIS VADEN]

Dartwick, I beg to differ.  If two bows of the same wt. shoot 180fps and one is more efficient than the other it will have less stored energy, That would make it  considerable easier to draw.  Since I am old and weak I love a bow that is easy to draw and gives the same performance.